COPEPOD PREDATION ON PHAGOTROPHIC CILIATES IN OREGON COASTAL WATERS

Clearance rates on natural assemblages of aloricate, phagotrophic ciliates (20 to 100 mum) were measured for calanoid copepods in Oregon, USA, coastal waters. Experiments were conducted prior to and throughout the 1991 upwelling season in an effort to determine the trophic significance of ciliates as copepod prey in a system often dominated by large phytoplankton. Copepods cleared ciliates at higher rates than they cleared phytoplankton, except during upwelling-induced diatom blooms. Clearance rates on ciliates (ml copepod-1 h-1) were measured for Calanus pacificus (12.6 to 32.4), Pseudocalanus sp. (4.8 to 7.4) and Centropages abdominalis (1.2 to 7.1). The highest clearance rates on ciliates were measured when the phytoplankton standing stock was <5 mug chl a l-1. No detectable clearance of ciliates was measured for Acartia longiremis or C. pacificus during upwelling bloom events (45 mug chl a l-1). Ciliate biomass ranged from 3 to 32 mug C l-1 and was significantly less than phytoplankton biomass (100 to 2200 mug C l-1) on all dates. During non-upwelling months and between diatom blooms, ciliates contributed 16 to 100 % of the estimated carbon ingested by copepods. Calculated ingestion of ciliate carbon (mug C copepod-1 d-1) averaged 6.5 (+/-2.4) for C. pacificus, 4.6 (+/-1.1) for Pseudocalanus sp. and 0.7 (+/-0.4) for C abdominalis. Ingestion of ciliates alone provided enough carbon to meet the basic respiration requirements of C. pacificus and Pseudocalanus sp. in January and February respectively. Estimates of predation, based on average copepod densities, indicate 25 to 45 % of the phagotrophic ciliate population is cleared from surface waters per day.